The present embodiments, in some embodiments thereof, relate to an image sensor based on pixels with reset state information, and more particularly, but not exclusively, to a method for selectively adjusting the integration time of each pixel, while permitting image sensor operation in global shutter mode.
Driven by the demand for low-power dissipation in state-of-the-art portable image systems, CMOS Active Pixel Sensors (APS) have become very attractive. CMOS imagers offer significant advantages in terms of low-power, low-voltage and monolithic integration, thus rivaling traditional charge-coupled devices (1-3).
Sensor dynamic range (DR) is one of the most important figures of merit in state of the art CMOS image sensors. The dynamic range problem exists when trying to capture scenes having a wide range of illumination. Generally, the illumination levels can range from 10−3 lux for night vision to 105 lux for scenes illuminated with bright sunlight. Even higher levels can occur with the direct viewing of light sources such as oncoming headlights. Bright scenes and wide variations in intra-scene illumination can arise in many situations, such as driving at night, photographing people in front of a window, observing an aircraft landing at night, and imaging objects for studies in meteorology or astronomy. The human eye has a dynamic range of about 90 dB and camera film of about 80 dB, but typical CMOS APS have a dynamic range of only 65-75 dB (4).
Dynamic range insufficiency of conventional video cameras is a serious problem in realizing a robust vision system for images having wide illumination conditions in the same scene. A narrow dynamic range results in the saturation of pixels with high sensitivity during exposure to high illumination levels. Saturation may result in the loss of part of the image information.
Generally, dynamic range is increased in two ways. The first method is noise reduction, which expands the dynamic range toward darker scenes. The second method is incident light saturation level expansion, which expands the dynamic range toward brighter scenes.
Different solutions for increasing the dynamic range in CMOS image sensors have been presented in recent years (5-13). A summary of existing solutions and their comparisons have been presented in O. Yadid-Pecht, “Wide dynamic range sensors”, Optical Engineering, Vol. 38, No. 10, pp. 1650-1660, October 1999, which is incorporated herein by reference in its entirety. Generally, these solutions can be divided into five groups:
(a) Companding sensors, such as logarithmic, compressed response photodetectors (e.g. 14);
(b) CMOS multi-mode sensors, where operation modes were changed according to light conditions (e.g. 15);
(c) Frequency based sensors, where the sensor output is converted to pulse frequency (e.g. 16);
(d) External control over integration time for the entire sensor or portions of the sensor, which can be further sub-divided to enable global control. Thus, the entire sensor can have different integration times for different frames, or different areas within the sensor can have different integration times (e.g. 13); and
(e) Autonomous control over integration time, in which the sensor itself provides the means for selecting different integration times (e.g. 11, 17, 19).
U.S. Pat. No. 6,831,689 by Yadid-Pecht., which is incorporated herein by reference in its entirety, presents a method and apparatus for expanding the dynamic range of a rolling shutter optical imager. The method entails individually controlling the integration time of each pixel of a sensor array, and providing a corresponding scaling factor for the electrical output of each individual pixel. The integration time of each pixel is controlled as a function of light intensity received by each individual pixel and resetting the pixel after a predetermined threshold for the output signal has been reached.
U.S. Pat. No. 6,977,685 by Acosta-Serafini et al., which is incorporated herein by reference in its entirety, presents an imager system provided in a semiconductor substrate. The imager includes a plurality of photosensitive, charge integrating pixels that are arranged in rows and columns of a pixel array, for capturing illumination of a scene to be imaged. Integration control circuitry is connected to access a selected pixel of the array for rolling shutter readout, and to generate pixel-specific integration control signals delivered to the selected pixel, independent of other pixels, based on dynamic range specification input provided by the input interface circuit.
Both U.S. Pat. No. 6,831,689 and U.S. Pat. No. 6,977,685 are applicable to imaging systems operating in rolling shutter mode.
A low-power wide dynamic range (WDR) sensor operating in global shutter mode (also denoted herein snapshot mode) is presented in “Low Power Global Shutter CMOS Active Pixel Image Sensor with Ultra-High Dynamic Range” (12), which is herein incorporated in its entirety by reference. Pixels are selectively reset at intermediate intervals during frame, in order to prevent saturation. An indication of whether the given pixel was reset at the end of the preceding interval is temporarily stored in the same output capacitance which stores the analog optical sensor level for readout.